After John Thaw, best known as television's Inspector Morse, died of cancer on 21 February 2002, Sheila Hancock, who had been married to him for 28 years, wrote in her diary: 'He was my whole life. Everything was in reference to him. Without him, I don't exist. I can't bear this crippling pain. I can't write it down.' But she has, since then, succeeded in writing it down. Her honest, graceful account leaves nothing out - Thaw's alcoholism, his depression, their separation, their love. The Two of Us - My Life with John Thaw is published on Wednesday.

Sheila Hancock's face smiles down on Liverpool, her eyebrows raised as high as they will go. In the poster above the Playhouse Theatre, she is larger than life and all celebration - a tilted glass of champagne in one hand, a cigarette holder in the other. Her ash blonde hair is short and sexy, her earrings incredible - like baroque foliage. It is a theatrical mask; it's unbelievable that she is 70. Tom Stoppard once described Hancock as a 'tired chrysanthemum who could put on a cruel blue look that could freeze a hummingbird to an oven door'. Decode that if you can. But her eyes do stop you in your tracks: exclamatory and ageless. It is typical of the woman she is that she can play a monstrous merry widow (in Bill MacIlwraith's The Anniversary ) with such fizz in the same week that her raw, stirring book is published.

I met her on a grey afternoon in a flat I almost needed Morse's help to find. In a sense, Sheila Hancock does not want to be found. She is 'hiding away' fighting a cold - Beecham's Powders at the ready - and worrying about her book. I first met her years ago and liked her greatly then. And whatever she was actually feeling (bloody awful, I suspect), she could not have been nicer on reacquaintance. Nor, in her fresh, flowery shirt - mauve and pink and cream - did she look less like the vulgar widow she brilliantly plays. But faces carry their recent history and there was a new sadness in her eyes. How could there not be?

During John Thaw's last illness, Sheila Hancock bargained with the devil. She would give up wealth (Thaw, she joked, would not approve), her career and die in agony to keep him alive. But the devil wasn't listening. And Sheila hadn't been on speaking terms with God since her mother and first husband, actor Alec Ross, died. (By the grimmest of coincidences, Ross and Thaw each died of cancer of the oesophagus.)

He never admitted he was dying. Four days before his death, Sheila recorded: 'John spent the morning choosing the colours and extras for his new Jag and discussing whether he should sign up for another year with Carlton. My heart is breaking.' And after a meeting with a 'palliative care man' who was 'utterly honest' with him, John turned to Sheila and said: 'Well, that all sounded very positive, didn't it, kid?' Hancock secretly met his doctor, urging him to be straight. 'But the doctor told me, "I am sorry, he just doesn't want to hear it and you have no right to make him treat it any other way."' Now she sees the courage in John's attitude, even though denial was never her thing.

After John's death, she longed for a book that could 'honestly tell me how ghastly grief is'. She has tried to write the book she wanted to read: 'I would have loved to read about somebody else, to realise I was behaving normally, not going mad.' She was 'consumed' with grief. An 'avalanche' of letters arrived. Most were 'mourning Morse'. They were not about John Thaw at all. There were condolences from people as various as a nun and a National Car Parks attendant. More alarmingly, she received a letter from a writer proposing a 'warts and all' biography, to go ahead without her approval, if necessary.

She knew what this meant. John's alcoholism was not public knowledge. She took advice and was told she should 'put a rumour around' to publishers that she was planning to write a book herself. She had tried to persuade John to write about himself but his reply was: 'Oh, nobody would be interested in me, kid.'

Sheila Hancock researched John's life as if he were a stranger and became engrossed: 'I almost forgot it was John,' She found out about his working-class childhood in Manchester (she grows up beside him, in alternate chapters, in her own happier working-class family in King's Cross, London and suburban Bexleyheath, Kent). Sheila and John's parallel lives continue as they get into Rada (at different times). Sheila describes John's hilarious audition as Richard III in a teddy boy suit. 'Mercifully,' she reflected, 'the panel had been shrewd enough to see the potential of the strange lad.'

John was a strange lad, known in the neighbourhood as 'the boy with the grey vest and no underpants'. He had a coconut of which he was inexplicably fond. But he had reason to be unusual. His mother had walked out on him and his brother when he was seven. 'He was polluted by what happened to him all his life.' He never forgave his mother. Sheila has more than forgiven her. She has tried to understand her. Oddly, it was seeing John's mother's furniture that was the breakthrough. Sheila's voice became excited as she recalled: 'It was when I saw these two shiny, garish cocktail cabinets. And a terrible converted lamp with a gold silk shade. I thought, I know this woman. This woman was John. This woman was a star. And life shat on her. She was all alone in a godawful house with two whining kids. She thought she could go off with this glitzy guy - because she was sparky, full of life. I could see where she was coming from. I thought I must put in a word for her somehow.'

She is a great defender, Sheila. A protester, too. She describes having a go at Tony Blair in the book. And she winces like a schoolgirl when I remind her of tigerishly assailing newspaper photographers in a post office who were trying to snap John Thaw during his last days. She has a crusading streak, too (she runs - and pours her earnings into - the John Thaw Foun dation, a charity for young people who need 'a helping hand'.)

But her book is not protective at the expense of truth. She wants to set the record straight and that means untwining Thaw and Morse. Morse was an intellectual; Thaw wasn't. Thaw had an amazing sense of humour; Morse didn't. But Morse and Thaw had some things in common: 'Shyness and a love of music. Like Morse, John had an awkwardness with women - because of what his mother did.' Both were workers; Thaw, at times, was a workaholic, always worrying about where the next job was coming from. Thaw loved the good life that Morse paid for - his houses (in London, Wiltshire and France), his swimming pool. Sheila does not need to add that Morse liked a drink, Thaw needed one. The little black bag he carried everywhere was full of vodka bottles.

John met Sheila co-starring in a play called So What About Love? He was grumpy, she was not impressed. But he found a charming way of letting her know he meant to be her friend. When her mother was diagnosed with cancer - the prognosis bad - John sat her down, put headphones on her head and played her Simon and Garfunkel's 'Bridge Over Troubled Water'. He would become her bridge. And she his.

When they married, they each had a daughter. Melanie was Sheila's by her first marriage, Abigail was John's. Together, they had a third, Joanna, when Sheila was 42. All three women are actresses. 'I have run the book past the girls - Jo particularly finds it very difficult.' But it was Abigail who criticised an early draft about the alcoholism saying: 'Mum, you have soft-pedalled on this. You should be more honest to show what a miracle it was when Dad came through and also to show how we all suffered.'

'Alcoholics are utterly dear one minute,' Sheila elaborated, 'but there is also a blanket hatred with which you cannot reason.' Alcohol was 'a terrible trigger that put John into an appalling decline'. It was a Harley Street doctor, Beauchamp Colclough, an unusual, wild-haired Irishman, who cured him. His therapist had said he was suffering from clinical depression but his moods improved dramatically once he became permanently sober. Alcoholics Anonymous helped Sheila: 'I remember my first meeting. I was distraught, not understanding what was going on in my life. Hearing these people, I thought, my God, other people are like me. This is an illness. I'm not going mad.' It is the second time she has expressed the need to have her feelings echoed. 'I can't do solitude,' she has written.

As we talked, there was drilling from the building site next door to her Liverpool flat beyond which the Mersey flowed brown and sluggish. Suddenly, she surprised me by announcing with energy: 'I know how to build a block of flats. I have watched them for hours building the foundations. I've watched bricks being laid, like Lego. I've seen workers sacked. There is always something to engross you in life.'

She has become interested in the slow, rowdy drama of it all. She loves cities. She prefers to be 'in the thick of it'. She is 'not one of those people who retire to the country'. She doesn't want to be put out to pasture and can't face Luckington, their beautiful house in Wiltshire. 'I feel John's absence so acutely there. There is nothing in the house that doesn't have a memory. I can only bear it when the grandchildren are there.' She has four: Jack and Lola (Melanie's), Molly-Mae and Talia (Abigail's), and Joanna is expecting a baby in November. She is surprisingly 'all right' in their house in France, which John Thaw adored. But she has sold their London house and moved into a flat in Hammersmith, by the river. 'I am back to my roots. I used to live above a pub [(the Carpenter's Arms in King's Cross]. Now I am near a pub again.'

She refuses to be a permanent mourner or a snail. She loves driving Mavis, the Jag John gave her. 'Mavis is glorious, a great joy in my life, very unsuitable.' Recently, she passed an advanced driving test. Why Mavis? It was an 'ordinary' name to 'bring a flash lady down to size'.

Unlike Mavis, Sheila needs no downsizing.'I have to say that since John died I feel older. I used to feel extremely attractive and sexy because that's how he saw me. When I look at myself in the mirror now, particularly when I am naked, I think how could he ever have looked at me and still thought I was so dishy? He must have been blind, but he did. Maybe his eyesight was going or something. I think it was because he didn't wear glasses'.

She laughs, then sounds serious again. 'But now I am conscious that certain things in my life are over. I will never have another man in my life. Let's be honest, the years of active life I have left are not that many. That is a fact. And I really want to take that on board.' She doesn't like the modern trend of pretending to be younger than we are. She has nothing against cosmetic surgery - it is just that she knows time for what it is.

As I leave Liverpool, I stop to look again at her picture above the theatre - such a brave face. Not long ago, a woman asked her: 'Weren't you Sheila Hancock?' She was. She still is.

· Two of Us by Sheila Hancock is published by Bloomsbury at £17.99. To order a copy for £17.09 with free UK p&p, call the Observer Book Service on 0870 836 0885, or go to observer.co.uk/bookshop